446 ZOOLOGY 



the irregular spaces in the tissues, in which the lymph collects, we find vessels less 

 regular than the veins, often running together and then rebranching, gradually 

 approaching the body cavity. On their route they pass through knots of special 

 tissue lymphatic glands, where colorless amoeboid cells are added. Special 

 lymphatics the lacteals gather food from the intestines and, uniting with the 

 general lymphatics, finally empty into the large veins in the neck region. The 

 escaped lymph is thus returned to the blood. 



459. The Respiratory Structures differ from those of birds 

 chiefly in the fact that they are confined to the anterior or 

 thoracic cavity, in which they hang freely, suspended by the 

 bronchi. There are no air-sacs outside the lungs, hence all 

 the air passages terminate in the alveoli, in the walls of which 

 are the pulmonary capillaries. Inspiration and expiration of air 

 is affected by increasing and decreasing the size of the chest cavity 

 by means of the muscles between the ribs and by the contraction 

 of the muscles of the diaphragm which is normally arched for- 

 ward into the chest. By its contraction the viscera are forced 

 backward and more space is given to the lung, which at once 

 fills the chest cavity as the result of air-pressure on the inside 

 of the lung. 



460. Nervous System. The special feature worthy of note 

 in the nervous system of mammals is the large size of the brain, 

 especially of the cerebral hemispheres. In the higher mammals, 

 particularly, these become complicated by folds and convolu- 

 tions by which the surface or cortex of the brain is much in- 

 creased. The brain cells, or gray matter of the brain, are espe- 

 cially abundant in the superficial part, and therefore this in- 

 crease of surface means that these cells are increased in amount 

 as compared with other vertebrates. The intelligence of an 

 animal is roughly proportional to the amount of the cortex. 

 The cortex is a thin layer, varying from one to five millimeters 

 in thickness. It has been estimated that there are about 9,000,- 

 000,000 nerve cells in the cortex of a human brain. These nerve 

 cells represent about one five- thousandth of man's total weight, 

 and yet they furnish the basis for the conscious life and the con- 

 trol of the body. The fibrous tracts connecting the various por- 

 tions of the cortex are likewise more perfectly developed among 

 the mammals, and most of all in man. 



